In the manufacture of sheet materials, it is well known that various sheet properties can be detected “on-line,” that is, while a sheet making machine is operating. On-line measurement devices measure sheet properties such as thickness, basis weight, moisture content, chemical composition and the like. Typically, such on-line devices employ sensors that periodically traverse, or scan, the moving sheets in the cross direction, which is perpendicular to the machine direction of sheet travel.
Visible, near-IR and mid-IR sensors share a common need for large spectral range, high spectral resolution and high signal-to-noise ratio. A large spectral range is needed for the sensor to address a wide number of applications whereas high spectral resolution and signal-to-noise insure sensor accuracy and repeatability. However, these sensor attributes are usually mutually exclusive. For example, a single detector and filter combination affords high signal-to-noise ratio and potentially good spectral resolution but does not provide adequate spectral range. Conversely, a compact spectrometer provides high spectral range and resolution but sacrifices throughput. Additionally, while spectrometers provide high spectral range and resolution, a single unit does not cover the entire range between the visible and mid-IR due to practical and technical considerations.
Plastic and paper industrial applications require versatile detectors with the above combination of characteristics for thickness measurements. Currently, a single sensor is employed to measure the thickness of thin plastic films on biax lines. The very thin films are measured using interferometry in the visible or near-IR where absorption is weak whereas the thicker films (>15-20 microns) are measured using adsorption further out in the near-IR.